This application is based on application No. 99-11043 filed in the Korean Industrial Property Office on Mar. 30, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing and, more particularly, to a positive active material for a rechargeable lithium battery, having good structural and thermal safety properties and a method of preparing the same.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provoded needs for a rechargeable battery having a high capacity as well as a light weight. From the viewpoint of the capacity improvement per unit weight, the rechargeable lithium battery is preferred because lithium has a high standard potential as well as a low electrochemical equivalent weight.
Rechargeable lithium batteries employ materials into or from which lithium can be intercalated or deintercalated as positive and negative active materials.
Metallic lithium has been used for the negative active material in rechargeable lithium batteries. However, lithium metal has good reactivity toward electrolyte and deposits to form a dendric layer which increases the reactivity of lithium. The problem of lithium reactivity toward the electrolyte has been addressed by replacing the lithium metal with carbon-based materials. Owing to the use of the carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as the reasonable shelf life.
A chalcogenide metal compound from or into which a lithium ion can be extracted or inserted, during a charge or discharge, is generally used for the positive active material. Alternatively, transition metal oxide-based compounds such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0 less than x less than 1) or LiMnO2 are generally used for positive active material. LiNiO2 has a high charge capacity, but is difficulty produced. Manganese-based material such as LiMn2O4 or LiMnO2, etc. is easiest to prepare, is less expensive than the other materials and has environmentally friendly characteristics, but has low capacities. LiCoO2 is widely used as it has an electrical conductivity of about 10xe2x88x922 to 1 S/cm at room temperature and high battery voltage, but has poor safety characteristics during high rate charges and discharges.
Generally, such composite metallic oxides are manufactured by the solid phase method. The solid phase method includes the steps of mixing with a solid raw material powder and sintering this mixture. For example, Japanese Patent publication No. Hei 8-153513 discloses a method in that Ni(OH)2 is mixed with Co(OH)2, or hydroxides including Ni or Co are mixed and the mixture is heat-treated, followed by the heat-treated mixture being ground and then the ground mixture sieved to produce LiNi1-xCoxO2 (0 less than x less than 1). In another method, LiOH, Ni oxide and Co oxide react and the reactant is initially sintered at 400 to 580xc2x0 C. and the sintered reactant is sintered a second time at 600 to 780xc2x0 C. to produce a crystalline active material.
However, in such conventional methods, the resulting active material has both low structural and thermal stability, reducing the safety of the battery.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery, having excellent structural and thermal safety properties.
It is another object to provide a method of preparing the positive active material.
These and other objects may be achieved by a positive active material for a rechargeable lithium battery including a cobalt-based compound selected from the group consisting of compounds represented by the formulas 1 to 4:
LiCoA2xe2x80x83xe2x80x83(1)
LiCoO2-xBxxe2x80x83xe2x80x83(2)
LiCo1-xMxA2xe2x80x83xe2x80x83(3)
LiCo1-xMxO2-yByxe2x80x83xe2x80x83(4)
where A is selected from O, S, F or P
B is selected from S, F or P
M is a transition metal selected from Al, Mg, Cr or Mn; Sr; or lanthanide metal selected from La or Ce, 0 less than x less than 1 and 0 less than y less than 1.
The compound includes secondary particles of 10 to 30 xcexcm in size gathered with primary particles with a size of 1 to 5 xcexcm. A surface of the active material is coated with metal oxide.
The present invention further includes a method of preparing the positive active material. In this method, a cobalt-based compound powder selected from the group consisting of formulas 1 to 4 is obtained. The compound includes secondary particles of 10 to 30 xcexcm in size and the secondary particle is gathered with primary particles with a size of 1 to 5 xcexcm. The powder of the compound is coated with a metal alkoxide solution or a metal aqueous solution to make a metallic alkoxide or metal-coated powder. The coated powder then heat-treated such that the metallic alkoxide or metal-coated powder is converted into a metallic oxide-coated powder.